1. Field of Invention
The invention relates to a system and method for representing compressed information within a data stream.
2. Description of Related Art
Currently, numerous standards exist for encoding data for transmission across a channel to a receiver (decoder). For example, the International Standard ISO/IEC 13818-7, Advanced Audio Coding, also known as MPEG-2 AAC, specifies a syntax for representing compressed audio signals which are suitable for transmission over limited bandwidth channels. The same syntax with one minor backwards compatible extension is International Standard ISO/IEC 14496-3, also known as MPEG-4 audio. Because these standards typically address transmission of the compressed data over bit-serial channels (e.g., a radio modem channel) the compressed format is often referred to as a bitstream, although the standard is equally valid for other transmissions or storage formats.
Current standards for representing compressed data in a data stream may not support re-synchronization of the bitstream and a decoder. Additionally, current standards may not provide for any explicit form of error detection, error control, or error recovery. For example with existing protocols, such as MPEG-2 AAC, it is necessary for the decoder to know the beginning of a received stream of raw data in order for the decoder to be able to decode the raw data. While information for synchronization of a decoder to a raw data stream is available in applications, such as receiving a computer file from a hard drive or the like, in many applications the synchronization information is not available. For example, in satellite or digital radio communications, the decoder can begin to receive the signal at any point during the signal transmission. Accordingly, the decoder needs to be able to synchronize with the signal in order to properly decode the signal.
One solution to resolving the synchronization problem, is to provide a separate synchronization channel in parallel to the data channel. An example of this technique could include having a decoder receive a first data stream containing video data, a second data stream containing audio data and a third data stream containing synchronization data. Accordingly, the decoder may then use the synchronization data to decode and synchronize with both the audio and video data. However, such systems can become very complicated since the decoder must receive and process numerous different signals.
Accordingly, there exists a need for providing a method for representing compressed information which provides for re-synchronization along with error detection and error correction.
In the Advanced Audio Coding (AAC) syntax, for example, the bitstreams which represent a contiguous interval in the signal time domain can be represented as a series of contiguous raw datablocks in the compressed time domain. The raw datablocks can include a series of elements, such as a single channel element (SCE), a channel pair element (CPE), a coupling channel element (CCE), a Ife channel element (LFE), a data stream element (DSE), a program config element (PCE) and a fill element.(FIL).
Of particular interest is the DSE, or data stream element, which is composed of a DSE ID, an element instance tag, a data byte align flag, a count, an Esc count, and a series of data stream bytes. For the DSE, the MPEG-2 AAC standard specifies the length and interpretation of the DSE ID, an element instance tag, the databyte align flag, the count and the ESC count. However, the MPEG-2 AAC standard only specifies the length, but not the interpretation of the data stream bytes. Furthermore, the value of count is equal to the number of data stream bytes that are present in the DSE. Accordingly, the MPEG-2 AAC syntax, and associated semantics, permits the DSE block to carry a number of bytes of information (i.e., xe2x80x9ccountxe2x80x9d occurrences of xe2x80x9cdata stream bytexe2x80x9d) whose meaning can be unknown to a standards compliant AAC decoder. By doing so the AAC standard is capable of being extended on a user-specified basis.
The present invention is a method for using the DSE block to give a bitstream of compressed data a re-synchronization, error detection, error control and/or error recovery capabilities while still maintaining full compliance to a standard, such as the MPEG-2 AAC standard. Accordingly, the present invention provides a system and method for storing re-synchronization, error correction and/or error detection data within an existing communication protocol, while still complying with the existing protocol""s requirements. By doing so, data information can still be passed from an encoder to a decoder via a channel using an existing and well known standard transport protocol. However, the existing well known transport protocol can now include the data necessary for synchronization of the decoder to the received raw data, along with error detection and error correction by the decoder.